The present invention is directed to the field of electrochemical capacitors, and particularly electrochemical supercapacitors. More specifically, the present invention is directed to a current collector for use in a double electric layer electrochemical supercapacitor electrode assembly.
There is an increasing focus on the use of capacitors as a means for storing electrical energy. These capacitors can efficiently store and redistribute a large amount of electrical energy. For example, such capacitors may be used: as a main power supply; as a back-up power supply; and for power quality assurance (e.g., to compensate for short-term power “surges”, “spikes”, and “skips” common to utility-supplied electric power). Such capacitors may also be used to provide load-leveling by storing an amount of electrical energy provided during off-peak hours and thereafter re-distributing said electrical energy during periods of peak demand. It is also possible to use such capacitors as a primary or secondary power source in situations where a portable source of power is required, such as with a variety of vehicles.
A double electric layer (DEL) capacitor typically comprises a pair of electrodes residing in a spaced apart relationship, between which resides an electrolyte. The electrolyte can be either aqueous or non-aqueous in nature, depending on the composition of the electrodes. A separator typically also resides in the space between the electrodes.
One or both of the electrodes in a DEL capacitor may store electrical energy through a double layer electrochemical mechanism. In a double electric layer storage process, a layer of electrons forms at the electrode side of the electrode/electrolyte interface. A layer of positive ions also forms on the electrolyte side of the electrode/electrolyte interface. The voltage across the electrode/electrolyte interface increases with charge accumulation, and is eventually released during discharge of the capacitor.
One or both of the electrodes of a DEL capacitor may generally be polarizable electrodes—although, it has been found that constructing a DEL capacitor with one polarizable electrode and one non-polarizable electrode provides the DEL capacitor with a specific energy capacity that is greater than that of a capacitor with two polarizable electrodes. The polarizable electrode may comprise, for example, an active material and a current collector to which the active material is affixed. The most commonly employed active material is one of a plurality of activated carbon materials.
Each of the electrodes of such a DEL capacitor is typically affixed by some means to a current collector. Current collectors are commonly constructed of a material that exhibits good electrical conductivity—typically a metal. As at least a portion of the current collector must reside in the electrolyte along with the electrode material, it must be ensured that the current collector material will not react adversely thereto. For example, the electrolyte of a DEL capacitor may consist of an aqueous sulfuric acid solution or some other aqueous or non-aqueous material. In such a case, it must be ensured that the electrolyte will not erode or corrode the current collector material, such as through an oxidation-reduction (redox) process.
Consequently, while various embodiments of DEL capacitor current collectors are known, each typically has one or more inherent disadvantages. For example, since different electrochemical capacitors utilize different electrolytes and different active mass materials, current collectors for use therewith should have certain corresponding electrochemical, physical, electrical, mechanical and processing characteristics. This has led to the need for a wide variety of different current collector materials.
Further, using materials such as sulfuric acid for the electrolyte in a DEL capacitor eliminates as an option a variety of inexpensive metals or alloys that could be used in constructing a current collector. Because such materials demonstrate low stability in the specified electrolyte, their use would significantly narrow the capacitor operating voltage window and result in a decrease in specific energy and power parameters of the capacitor. Thus, DEL capacitors have commonly employed current collector materials that exhibit better stability in such electrolytes, which materials are typically expensive to procure. Such materials may include, for example, Ti, Al, Ni, Ag, Nb, Ta, W and a variety of alloys thereof.
It is possible to use more inexpensive materials such as steel and similar metals to form a current collector of a DEL capacitor. However, because such metals are not sufficiently resistant to certain electrolytes (e.g., sulfuric acid electrolytes), known DEL current collector designs making use of such metals have also required the use of a protective coating that is resistant to the electrolyte used in the capacitor. Without the protective coating, a current collector comprised of steel or a similar metal will degrade in the presence of a sulfuric acid electrolyte—such as by corrosion. Current collector corrosion can have a negative effect on the cycling capacity and service life of a capacitor.
As one example of a known design of this nature, a steel current collector may utilize a protective layer of graphite foil. While this and other similar coating materials may offer acceptable resistance to the electrolyte in which they reside, there has been a great deal of difficulty in obtaining adequate adhesion between such protective coatings and the subjacent electrode materials. As a result, the electrolyte eventually intrudes between the protective coating and the current collector, whereafter corrosion of the current collector material occurs.
It should be realized that any degradation or erosion of such a metal current collector can adversely effect performance of a DEL capacitor. For example, when a sulfuric acid electrolyte is used, even substantially insignificant quantities of iron present therein can harshly decrease the decomposition voltage of the electrolyte and result in a significant reduction in the operating voltage of the capacitor. Hence, degradation of the current collector should be avoided.
As can be understood from the foregoing discussion, there are several disadvantages associated with known DEL capacitor current collector designs. Thus, current collectors of the present invention utilizes an improved design that substantially reduces or eliminates many of the problems associated with known current collectors.